• Steel and Composite Structures
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• v.22 no.6
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• pp.1487-1505
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• 2016

This paper presents the results of an experimental study on the behavior of a new type of composite FRP-concrete-steel member subjected to bi-axial eccentric loading. This new type of composite member is in the form of concrete-filled square steel tube slender columns with inner CFRP (carbon fiber-reinforced polymer) circular tube, composed of an inner CFRP tube and an outer steel tube with concrete filled in the two tubes. Tests on twenty-six specimens of high strength concrete-filled square steel tube columns with inner CFRP circular tube columns (HCFST-CFRP) were carried out. The parameters changed in the experiments include the slenderness ratio, eccentric ratio, concrete strength, steel ratio and CFRP ratio. The experimental results showed that the failure mode of HCFST-CFRP was similar to that of HCFST, and the specimens failed by local buckling because of the increase of lateral deflection. The steel tube and the CFRP worked together well before failure under bi-axial eccentric loading. Ductility of HCFST-CFRP was better than that of HCFST. The ultimate bearing capacity of test specimen was calculated with simplified formula, which agreed well with test results, and the simplified formula can be used to calculate the bearing capacity of HCFSTF within the parameters of this test.

• Steel and Composite Structures
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• v.16 no.5
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• pp.455-480
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• 2014

Sixteen concrete filled square CFRP-steel tubular (S-CFRP-CFST) stub columns under axial compression were experimentally investigated. The experimental results showed that the failure mode of the specimens is strength loss of the materials, and the confined concrete has good plasticity due to confinement of the CFRP-steel composite tube. The steel tube and CFRP can work concurrently. The load versus longitudinal strain curves of the specimens can be divided into 3 stages, i.e., elastic stage, elasto-plastic stage and softening stage. Analysis based on finite element method showed that the longitudinal stress of the steel tube keeps almost constant along axial direction, and the transverse stress at the corner of the concrete is the maximum. The confinement effect of the outer tube to the concrete is mainly focused on the corner. The confinements along the side of the cross-section and the height of the specimen are both non-uniform. The adhesive strength has little effect both on the load versus longitudinal strain curves and on the confinement force versus longitudinal strain curves. With the increasing of the initial stress in the steel tube, the load carrying capacity, the stiffness and the peak value of the average confinement force are all reduced. Equation for calculating the load carrying capacity of the composite stub columns is presented, and the estimated results agree well with the experimental results.

• Steel and Composite Structures
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• v.38 no.2
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• pp.165-176
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• 2021

Existing research on confined concrete filled steel tubular (CCFT) columns has been mainly focused on static or cyclic loading. In this paper, square section CCFT and CFT columns were tested under both static and impact loading, using a 10,000 kN capacity compression test machine and a drop weight testing equipment. Research parameters included bonded and unbonded fiber reinforced polymer (FRP) wraps, with carbon, basalt and glass FRPs (or CFRP, BFRP, and GFRP), respectively. Time history curves for impact force and steel strain observed are discussed in detail. Experimental results show that the failure modes of specimens under impact testing were characterized by local buckling of the steel tube and cracking at the corners, for both CCFT and CFT columns, similar to those under static loading. For both static and impact loading, the FRP wraps could improve the behavior and increase the loading capacity. To analyze the dynamic behavior of the composite columns, a finite element, FE, model was established in LS-DYNA. A simplified method that is compared favorably with test results is also proposed to predict the impact load capacity of square CCFT columns.

• Steel and Composite Structures
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• v.47 no.6
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• pp.781-794
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• 2023

In order to directly apply seawater and sea sand in construction without desalination, a type of square concrete-filled steel tube (CFST) encased with prefabricated seawater sea-sand concrete filled Polyvinyl Chloride (PVC)/Glass Fiber Reinforced Polymer (GFRP) tube column was proposed. Twenty short columns were tested under uniaxial loads, and the test parameters included inner tube types, seawater sea-sand concrete replacement ratios, concrete strength, the wrapping area of Carbon Fiber Reinforced Polymer (CFRP) strips and the thickness of GFRP tube. The effects of the parameters on failure modes, loading capacity, ductility and strain responses were discussed. All the tested specimens failed with serious buckling of the steel tubes and fracture of the inner tubes. The specimens had good residual bearing capacity corresponding to 64% to 88.9% of the peak capacity. The inner GFRP tubes and PVC tubes wrapped by CFRP strips provided stronger confinement to the core concrete, and were good choices for the proposed columns. Moreover, an analytical model for the composite column with different inner tube types was proposed.

• Steel and Composite Structures
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• v.29 no.2
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• pp.187-200
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• 2018

This paper handles the repairing of deficient square Concrete-Filled Steel-Tube (CFST) beams subject to bending through an experimental and numerical program. Eight square-CFST beams were tested. A 5-mm artificial notch was induced at mid-span of seven beams, four of them were repaired by using CFRP sheets and two were repaired by using GFRP sheets. The beam deflection, strain and ultimate moments were recorded. It was found that providing different cut-off points for the different layers of FRP sheets prohibited failure at termination points due to stress concentrations. Using different lengths of FRP sheets around the notch retarded crack propagation and prevented FRP rupture at the crack position. Finite element analysis was then conducted and the proposed FE model was verified against the recorded experimental data. The influence of various parameters as FRP sheet length, tensile modulus and the number of layers were studied. The moment capacity of damaged square-CFST beams was improved up to 77.6% when repaired by using four layers of CFRP, however, this caused a dramatic decrease in beam deflection. U-wrapping of notched-CFST beam with 0.75 of its length provided a comparable behaviour as wrapping the full length of the beam.